Prior neutron radiography inspection devices, such as that disclosed in U.S. Pat. No. 4,300,054, to the inventor of the present invention, incorporate as the radiation source an ion accelerator neutron generator which generates 14 MeV neutrons. This neutron generator is a high-flux sealed-tube unit which derives neutrons from the reaction which occurs when a tritium target is bombarded by a beam of deuterium ions. This reaction is denoted by .sup.3 H(d, n).sup.4 He, and is commonly abbreviated "D-T".
The radiation source is housed in an essentially spherical inspection head containing a hydrogen-rich liquid moderator. The head contains a collimator mounted with one end within the liquid moderator. The 14 MeV neutrons, designated as fast neutrons, produced in the tritium target, are moderated or thermalized by the liquid moderator surrounding the source. These thermalized neutrons are then directed by the collimator to the structure which is to be inspected. The inspection head is maneuverable on support arms such that difficult-to-reach structures and assemblies may be examined in the field or during manufacture.
The minimum size and weight of the inspection head are dictated primarily by the energy of the source neutrons and the volume of moderator material required to reduce the energy of the neutrons to thermal levels, measured at the input of the beam collimator, necessary for thermal neutron radiography. The thermal level is approximately 0.025 eV. Additional moderator volume greater than that required for effective thermalization of the fast neutron beam from the source serves to reduce radiation levels around the device, and hence reduces the shielding and/or the distance an operator or other personnel must stand away from the device during operation. However, the addition of moderator material beyond the minimum required for effective thermalization rapidly increases the weight of the inspection head and decreases its practical maneuverability. Thus, in the design of a system using an accelerator source of 14-MeV neutrons, the requirement for moderator material and shielding determines the physical size of the inspection head and thus controls the maneuverability of the device.
Although the prior art device identified above provides a maneuverable neutron radiography inspection device, the ability to reduce the size of the inspection head even further would increase maneuverability and make the unit useable in more situations, for example, those requiring extended reach, than is presently possible.